I do not understand this line;"Part of the oil ring was in the coolant
jacket ". I guess you meant the O-ring? i.e. the upper green one? I always
wondered as to the real purpose of these since the flat, metal ring does all
the sealing.
   Before attaching the heads I like to check them for trueness. I use the
respective cylinder sleeve and some Prussian Blue to ensure 100% contact. I
rub a very thin layer on the tops of the sleeves then carefully twist them
about 30 degrees in contact with the heads sealing surface. Any low or high
spots will be revealed by the absence or presence of the blue.
  If not perfect I use valve grinding paste to lap the sleeve into the head.
Clean then recheck with blue. Repeat until the sleeve leaves a perfect blue
circle in the head.
   If I am only taking the heads off I use an old cylinder. This technique
is an old air cooled engine trick.
Ken Lewis
http://neksiwel.20m.com/